1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a unique design of a soluble burnable absorber rod for use in a nuclear reactor which achieves substantially complete absorber burnup and has reduced fabrication cost.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of transverse grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core.
The overall advantages to be gained in using burnable poison at stationary positions in a nuclear reactor core are described in U.S. Pat. No 3,510,398 to Wood. Heretofore, rods containing burnable poison intended to be stationarily positioned within the reactor core have been of the "fixed" type. By a rod being of the fixed type, it is meant that the absorber content of the burnable poison at any axial elevation on the rod is fixed by the initial loading of the material during manufacture of the rod.
The burnable poison rod in the Wood patent is representative of the fixed type. A major disadvantage of the fixed type absorber rod, such as the one illustrated and described in this patent, is that not all of the poison material in the rod burns up completely or depletes evenly. The shape of the axial depletion curve for the fixed type absorber rod is approximately the same as the axial neutron flux distribution curve averaged over the core life cycle. However, because of the lack of correspondence between the average axial distribution of neutron flux and some of the neutron flux peaks occurring in the reactor core over the life cycle of core operation, poison material at certain axial locations of the fixed type burnable poison rod depletes more rapidly than at other locations. This results in incomplete absorber depletion at the other locations, which means there is a substantial residual absorber penalty at the end of the cycle.
Consequently, a need exists for a burnable poison rod design which will have an improved fuel cycle cost benefit over the previous fixed design, as represented by the rod design in the Wood patent, in terms of fabrication costs and increased length of core cycle.